1. Field of the Invention
This invention relates generally to geophysical data acquisition systems, and in particular to a geophysical field acquisition system with circuitry which increases effective system bandwidth. Still more particularly, the invention relates to a geophysical acquisition system having circuitry which may be configured by operator control during seismic exploration operations to increase the effective bandwidth of the combination of earth acting as a filter to seismic signals and the geophysical acquisition system.
2. Description of the Prior Art
Recording systems for providing a permanent record, typically on magnetic tape, of geophysical data acquired in field operations are well known. For the purpose of this disclosure the term "geophysical data" includes data acquired in a land prospecting environment, where the sensors may be seismometers, geophones, or other types of sensors, as well as data acquired in a marine prospecting environment where the sensors may be hydrophones or the like. A typical digital field recording system is that disclosed in U.S. Pat. No. 3,819,864.
Various designs of geophysical acquisition systems (also called seismographs and field acquisition systems) exist, the simplest being fixed systems where amplifier gain is preset and constant. Such prior art systems have a preamplifier stage, an analog gain stage, low-cut filters, notch filters, and high-cut anti-alias filters for each channel. A multiplexer switches the data channel sequentially to an analog to digital (A/D) converter which outputs a binary signal proportional to the input voltage.
An important parameter of such geophysical acquisition systems is their dynamic range. Dynamic range is defined as the difference in DB between the largest input signal that can be processed and recorded, i.e., the signal having an amplitude just below that which would cause saturation in some element of the system, and the smallest signal which can be properly recorded by the system, generally considered to be a signal to noise ratio of 1. Dynamic range is commonly expressed as the ratio of the largest to the smallest signal and is typically expressed in decibels or dB.
The number of steps the A/D converter breaks the signal into defines the precision of the system. If it is broken into 12 bits, the system is more precise than if it is broken into 8 bits. An A/D converter is designed to accept a certain maximum voltage. It is important to amplify seismic signals (geophysical data) until the maximum voltages applied to the A/D converter are as close to (but not greater than) that value as possible. Using as many of the bits of the digital word as possible increases its significance. The significance of the digital word relates to the number of bits recorded above the system noise level. In other words, to take full advantage of the geophysical acquisition system, from a "significance" point of view, the signal must be amplified before the A/D stage with the gain of the amplifier being set as high as possible without saturating the electronics. The electronics become saturated when the largest voltage applied to the A/D converter is too big for converter full scale.
At any specific gain setting, a digital system can operate only within a defined input voltage range without the signal being either below the noise level or above saturation. The minimum detectable voltage is the larger of the threshold voltage for the least significant bit or the system noise level. The maximum detectable voltage is the level at which the A/D converter becomes saturated and outputs its largest possible digital number. Signals above the maximum level are truncated, either in their analog state or after conversion to digital form.
The systems typically used for petroleum exploration are designed to adjust their gains automatically according to the incoming signal and also to record the gain setting. These are floating-point systems where the number is recorded in exponent-mantissa scientific notation. The output from the A/D converter is the mantissa and the gain setting is the exponent. Binary gain systems were the first of these systems; an example is Texas Instruments' DFS III which is gain ranged in 6 dB increments (single bit shifts). The digital number is recorded as a 15-bit word and the gain is recorded in a 4-bit exponent. With binary gain systems, several (from 15 to 60 ) calls to shift are required before gain is increased. A single call is needed to decrease gain.
The next generation of gain ranging amplifier recording systems was the instantaneous floating-point (IFP) system. Only one call is required to increase gain by any amount. The Texas Instruments DFS IV and DFS V are examples of such instruments. The gain increment for these instruments is 12 dB (2-bit shift) but could have other step ratios as well, such as 2:1 (6 dB) and 8:1 (18 dB).
When the processing of the digital signals is accomplished using digital filters and the like, the effective dynamic range of a signal that can be recorded from a maximum significance point of view, is less than the actual dynamic range of signals applied to the A/D converter. For a 16 bit A/D converter (15 bit mantissa, 1 sign), the effective dynamic range of the A/D converter and digital processing elements of the system is between 40 and 50 dB.
When an explosive source generates a seismic signal into the earth, the signal is actually a composite signal or sum of many different oscillatory signals or waves which propagate into the earth and are reflected or refracted back toward the geophones of the geophysical acquisition system. The spectrum of the generated signal depends on the weight and type of explosive, its shape and the material around it. In typical land seismic explorations, the energy peak of the generated spectrum is in the 10 to 30 Hz range, with a typical value of 15 Hz. At frequencies higher than the peak, the energy decreases at about 6 dB per octave. The shape of the generated energy spectrum of compressed air ("air gun") and mass impact devices is similar to that of an explosive source.
The spectrum of vibratory sources (e.g., Vibroseis, a trademark of Conoco, Inc.) can be controlled by the use of nonlinear sweep signals. The high frequency energy of resulting source seismic signals can be increased.
For any spectrum of seismic waves input into the earth by one of the sources mentioned above, the earth responds as an attenuation filter. The frequency components of multiple frequency seismic waves have a propagation velocity which is about the same for any frequency, but each frequency of the composite wave is attenuated at about 0.5 dB per its wavelength. The number of dB of attenuation for each component of the wave doubles with each doubling of frequency. A doubling of the attenuation as measured in number of dB is equivalent to an exponential increase in attenuation. Thus, as a function of frequency, the seismic waves (having a generated energy spectrum) are attenuated exponentially with increases in frequency.
The waves propagating through the earth are also attenuated exponentially as a function of time. As a result, the attenuation spectrum of the earth (considered as a signal filter) changes exponentially with time and frequency. At any specific time, the energy spectrum of the returning waves is acted on by the "earth filter" and sensors which typically reaches a maximum level in a frequency range from about 10 to 40 Hz and then decays exponentially with frequency. Considering the generated spectrum of frequencies, as acted on by the "earth filter" and sensor at any particular time after source initiation, is like taking a "snapshot" of the energy content of returning waves from a certain depth in the earth, considering that the propagation velocity of elastic waves is fairly well known. The later in time the snapshot is taken, the deeper the "look" into the formation.
In view of the "effective dynamic range" of the A/D converter and digital processing systems of a modern geophysical acquisition system, the "effective system bandwidth" of the combination of the "earth filter" and the seismic acquisition system can be approximated. At any time after seismic energy is applied to the earth, the maximum signal amplitude of the spectrum is determined, recognizing that the gain of the gain ranging amplifier is set to produce a near maximum signal level to the A/D converter. The effective dynamic range, in dB, is then subtracted from the level of the maximum signal level in dB to produce a signal level in dB, below the maximum signal level, to establish an attenuation level of the earth-acquisition system, below which signals can not be measured with full "significance" as defined above. That lower signal level, or significance level, then allows determination of the effective earth-acquisition system bandwidth, defined as the band of seismic signals which may be recorded with full significance. The effective bandwidth so defined changes with each snapshot or time after the seismic signals are generated and input into the earth.
It is important that the effective bandwidth of the earth-acquisition system be as wide as possible so that earth layers of smaller relative thickness may be resolved. Resolution may be defined as the limit at which two features can be distinguished from the effects of one feature. An acquisition system which enhances the effective bandwidth of the earth-acquisition system simply has more earth layer resolving capability than one with less effective bandwidth.
Low-cut filters have been used in the prior art to enhance effective system bandwidth. Knapp and Stepples describe a cut-off frequency of 80 Hz, 24 dB/octave low-cut filter prior to digitizing for a geophysical acquisition system in GEOPHYSICS, Vo. 51, No. 2 (Feb. 1986) at page 288. Knapp and Steeples suggest that the filter cut-off frequency should not be so high and the roll off slope so steep as to filter away all of the signal, but it is also important that it be high enough to attenuate high-amplitude, low frequency signal and low-frequency noise that might mask low amplitude signals.